Load Transfer Mechanism to Offshore Jackets During Pile Driving

ABSTRACT Dynamic loads induced by pile driving during Qffshore installation can cause fatigue damage to secondary and primary jacket members. To check the fatigue performance of these members, a time domain fatigue analysis is required. The big unknown in performing such analysis is the input forcing function. The mechanism by which these loads are transferred to the jacket is complex and not fully understood. In this paper, a mechanism of load transfer is presented. A simplified analytical model to compute the magnitude and shape of the input force is developed. This model considers the effect of hammer type, pile geometry, pile stick-up length, leg batter, guide location, and friction. A parametric study was performed to examine these parameters on the force transferred to the jacket. Pile driving forces were determined for two piles and two hammers. Results generated by this model are compared well to measured data of a North Sea jacket monitored during pile driving. The simplified model is extended to compute pile dynamic stresses during driving. Results compared well with inelastic finite element analysis. 1. 0 INTRODUCTION Pile driving during jacket installation was shown to cause significant dynamic response [1,2]. Loads transferred to the jacket can become a dominant design factor for both main joint and secondary joints such as sacrificial anodes, hand rails and grout lines. The mechanism by which these loads are transferred to the jacket is complex and is not fully understood. In Reference [1], a trapezoidal forcing function was calibrated from measurements and used successfully in the analysis. This force was derived for a specific hammer (MRBS 8000), pile make up, and jacket geometry. Only the leg batter was introduced as a multiplier to modify the amplitude of the forcing function. In' this paper, a mechanism of load transfer based on pile lateral vibrations is presented. A simplified analytical model to compute the magnitude and shape of the input force is developed. This model considers' the effect of hammer type, pile geometry, pile stick-up length, leg batter, guide location, and friction. 2.0 FACTORS AFFECTING LOAD TRANSFER An analytical model based on pile lateral vibrations is proposed. Lateral vibration is induced by misalignment and selfweight. As a result, a normal force component is transferred to the jacket guide. This component is responsible for generating the friction force between the pile and jacket and inducing the jacket motion. This model is applicable for above water driving (Figure 1) and underwater driving (Figure 2). Different factors that contribute to the force transfer are discussed as follows: 2. 1 Hammer Type and Energy Significant dynamic response is associated with using large hammers such as the Menck MRBS 12500, Menck MHU 1700 [2] and Menk MRBS 8000 [1]. The impact diagram is usually determined using wave equation simulation. the impact diagram (force time history) for big hammers is characterized by higher peak force Fp and longer pulse duration t1[3].